One type of semiconductor device fabrication process is known as photolithography, or as just lithography. In photolithography, a material such as photoresist, which can also be referred to as just resist, is exposed to a light source through a photomask, which can also be referred to as just a mask. The material that is exposed to light is ultimately removed, such that the material that is not exposed to the light is not removed. Alternatively, the material that is not exposed to the light may be ultimately removed, such that the material that is exposed to the light is not removed.
To fabricate semiconductor devices having smaller and smaller features, the photomasks corresponding to the designs for such semiconductor devices may be modified so that photolithography is properly performed. One type of modification is known as optical proximity correction (OPC). OPC is a photolithography enhancement technique used to compensate for image errors due to diffraction or process effects. For instance, projected images may appear with irregularities such as line widths that are narrower or wider than designed, rounded corners instead of squared corners, and so on.
Conventional OPC corrects these errors by moving mask edges in a counter-biasing manner, until sufficient counter-bias is accumulated such that the image irregularities are determined to be approximately cancelled under some nominal set of imaging conditions. The mask may also be modified by adding extra polygons or other features in sparse areas of the pattern that is written on the photomask, so that each region of the mask contains patterns with similar spacings and densities. These extra features are referred to as assist features, and tend to reduce the variability in printed features as process conditions fluctuate. The conventional adjustment of polygon edges and insertion of assist features are both referred to herein as conventional OPC.
Another type of modification is source mask optimization (SMO), which simultaneously optimizes both the light source and the photomask shape. SMO is more complicated than simply adding assist features to photomask designs or moving edges within the photomask as is achieved in conventional OPC. In OPC, photomask designs are modified in accordance with a feedback counter-biasing method, or according to rules for assist feature deployment. By comparison, in SMO, more complex modifications are made to photomask designs in order to optimize the mask patterns, and simultaneously to optimize the light source pattern that illuminates the mask. In one particular SMO approach, an optimal set of light source and optical mask pattern are determined which define preferred illumination and imaging waves within a lithographic exposure process by using optimization techniques. At this point, optical mask pattern is ideally represented in frequency domain or continuous values. Wavefront engineering is then used to determine a set of actual manufacturable mask patterns that can produce the optimal imaging waves when the mask patterns are illuminated by the optimal illumination beams.
As such, wavefront engineering refers to the generation of manufacturable mask shapes that, when illuminated from some point in an exposure source, produce a specified optical wave within the light beam that is collected by a lithographic exposure tool. The waves can be specified on any plane or surface since it is known how to propagate waves from one surface to another. Two natural choices are to specify the waves in the pupil of the projection lens of the exposure tool (the waves being, for example, diffraction order amplitudes), or as field amplitudes at points on the exit face of the mask containing the mask shapes. In the latter case, the waves may be understood as a map of the transmission of an equivalent continuous mask, which may be in pixel values.